METABOLISME
INHOUD
1.1 Inleiding.................................................................................................................................................... 12
1.1.1 Definities .............................................................................................................................................. 12
1.1.2 Korte geschiedenis van de glycolyse ................................................................................................... 12
1.1.3 Glucose als brandstof molecule ........................................................................................................... 12
1.1.4 Fermentatie versus complete oxidatie ................................................................................................ 12
1.1.5 Lot van glucose in verschillende weefsels ........................................................................................... 13
Rode bloedcellen ........................................................................................................................................... 13
Hersenen ....................................................................................................................................................... 13
Hart en spier.................................................................................................................................................. 14
Vetweefsel .................................................................................................................................................... 14
Lever .............................................................................................................................................................. 14
1.1.6 De glycolyse is een katabole pathway ................................................................................................. 15
1.2 Opname van glucose in de cel via transporters ....................................................................................... 15
1.2.1 Gefaciliteerde diffusie: Glucose Transporters (GluT) .......................................................................... 15
1.2.2 Actief transport van glucose: Na+/glucose symporters ....................................................................... 16
1.3 De glycolyse ............................................................................................................................................. 16
1.3.1 Fase 1 glycolyse: Hexokinase ............................................................................................................... 16
Phosphoglucose isomerase ........................................................................................................................... 17
phosphofructokinase (PFK) ........................................................................................................................... 18
1.3.2 Fase 2 glycolyse: Het 6-C suiker wordt gesplitst in 2 3-C fragmenten door aldolase .......................... 18
1.3.3 Fase 3 glycolyse: Energie transformatie .............................................................................................. 18
1.3.4 Het behoud van de redox balans ......................................................................................................... 20
1.3.4.1 het lot van pyruvaat .................................................................................................................... 20
1.3.4.2 ethanol fermentatie .................................................................................................................... 21
1.3.4.3 lactaat fermentatie ..................................................................................................................... 21
1.3.5 Disacchariden als bron van koolhydraten ........................................................................................... 22
1.3.5.1 Galactose metabolisme .............................................................................................................. 22
1.3.5.2 Fructose metabolisme in de lever............................................................................................... 22
1.3.5.3 Overzicht ..................................................................................................................................... 23
1.3.6 Metabole afwijkingen .......................................................................................................................... 23
1.3.4.1 Lactose intolerantie........................................................................................................................... 23
1.3.4.2 Galactosemia’s ............................................................................................................................ 23
1.3.4.3 Fructose intolerantie en aldolase B ............................................................................................ 24
1
,1.4 De glycolytische weg is strikt gecontroleerd ............................................................................................ 24
1.4.1 Duale rol van de glycolyse ................................................................................................................... 24
1.4.2 Enzymen die belangrijk zijn in de regulatie ......................................................................................... 24
1.4.2.1 Situatie in de spier ...................................................................................................................... 25
1.4.2.2 Situatie in de lever ...................................................................................................................... 26
1.4.3 Kanker en glycolyse ............................................................................................................................. 27
1.5 De gluconeogenese .................................................................................................................................. 27
1.5.1 Noodzaak van de gluconeogenese ...................................................................................................... 27
1.5.2 Precursoren voor de gluconeogenese ................................................................................................. 27
1.5.3 Reacties van de gluconeogenese ......................................................................................................... 28
1.5.3.1 Pyruvaat carboxylase .................................................................................................................. 28
1.5.3.2 PEPCK: phosphoenolpyruvaat carboxykinase ............................................................................. 30
1.5.3.3 Omzetting van glycerol naar dihydroxyacetone phosphate ....................................................... 31
1.6 Gluconeogenese en glycolyse worden reciprook geregeld ..................................................................... 32
1.6.1 Glycolyse en gluconeogenese zijn niet tegelijk actief.......................................................................... 32
1.6.2 De balans tussen glycolyse en gluconeogenese in de lever is gevoelig aan de bloed glucose
concentratie ...................................................................................................................................................... 34
1.6.3 Substraat cyclering versterkt de reciproke signalen en produceert warmte door groot ATP verbruik36
1.6.4 Lactaat gevormd door samentrekkende spieren wordt gebruikt door andere organen: Cori cyclus . 36
2.1 Inleiding.................................................................................................................................................... 38
2.2 Omzetting van pyruvaat naar acetyl-CoA ................................................................................................ 39
2.2.1 Het pyruvaat dehydrogenase complex (PDH complex) ....................................................................... 39
2.2.2 Structuur acetyl-CoA............................................................................................................................ 39
2.2.3 Werkingsmechanisme pyruvaat dehydrogenase complex .................................................................. 39
2.2.3.1 E1 : TPP afhankelijke decarboxylatie (rate-limiting step) ........................................................... 40
2.2.3.2 E1: Oxidatie ................................................................................................................................. 40
2.2.3.3 E2: Vorming van acetyl-CoA ........................................................................................................ 41
2.2.3.4 E3: re-oxidatie van dihydrolipoamide ......................................................................................... 41
2.2.3.5 Overzicht ..................................................................................................................................... 41
2.2.4 Structuur van het pyruvaat dehydrogenase complex ......................................................................... 42
2.3 Reacties van de citroenzuurcyclus ........................................................................................................... 42
2.3.1 Citraat synthase vormt citraat uit oxaalacetaat en acetyl-CoA ........................................................... 43
2.3.1.1 Citraat synthase: werkingsmechanisme ..................................................................................... 43
2.3.2 Cis-Aconitase: citraat wordt geïsomeriseerd tot isocitraat ................................................................. 44
2.3.2.1 Cis-Aconitase: werkingsmechanisme .......................................................................................... 44
2.3.3 Isocitraat dehydrogenase : oxidatieve decarboxylatie van isocitraat ................................................. 44
2.3.4 α-ketoglutaraat dehydrogenase complex ........................................................................................... 45
2
, 2.3.5 Succinyl-CoA synthase ......................................................................................................................... 45
2.3.5.1 Succinyl-coA synthase: werkingsmechanisme ............................................................................ 45
2.3.6 Succinaat dehydrogenase .................................................................................................................... 46
2.3.7 Fumarase ............................................................................................................................................. 46
2.3.8 Malaat dehydrogenase ........................................................................................................................ 47
2.4 Regulatie van de citroenzuurcyclus ......................................................................................................... 47
2.4.1 Regulatie van het pyruvaat dehydrogenase complex ......................................................................... 47
2.4.1.1 Allosterisch: productinhibitie ...................................................................................................... 48
2.4.1.2 Covalente modificatie ................................................................................................................. 48
2.4.2 Regulatie van de citroenzuurcyclus ..................................................................................................... 48
2.5 De citroenzuurcyclus als bron van biosynthetische precursor moleculen: amfibool karakter ................ 49
2.6 Metabole aandoeningen .......................................................................................................................... 50
3.1 Inleiding.................................................................................................................................................... 51
3.1.1 Nood aan ATP regeneratie................................................................................................................... 51
3.1.2 Oxidatieve fosforylatie gebeurt in de mitochondriën ......................................................................... 51
3.2 Oxidatieve fosforylatie is afhankelijk van elektronentransport ............................................................... 52
3.2.1 De redox potentiaal en verandering in vrije energie ........................................................................... 52
3.2.2 ∆G voor transport van één H+.............................................................................................................. 53
3.3 De elektronentransportketen .................................................................................................................. 53
3.3.1 De respiratie keten .............................................................................................................................. 53
E’° van de verschillende complexen .............................................................................................................. 53
Ubiquinone (Q) (of CoQ) ............................................................................................................................... 54
Ijzer-zwavelstof centra worden veel gebruikt in de ETK ............................................................................... 54
3.3.2 Complex I: NADH-Q oxidoreductase.................................................................................................... 54
3.3.3 Complex II: Succinaat-Q reductase ...................................................................................................... 55
3.3.4 Complex III: Q-cytochroom c oxidoreductase ..................................................................................... 55
3.3.5 Complex IV: cytochrome c oxidase ...................................................................................................... 57
3.3.6 Toxische derivaten van O2 ................................................................................................................... 58
3.4 De protonengradiënt drijft de synthese van ATP..................................................................................... 58
3.4.1 Verband tussen elektronen transport en ATP synthese ...................................................................... 58
3.4.2 ATP synthase ....................................................................................................................................... 59
3.4.2.1 ATP synthase: Structuur .............................................................................................................. 59
3.4.3 Binding-change mechanisme van Paul Boyer ...................................................................................... 60
3.4.4 De protonen die langs de c-ring stromen zorgen voor ATP synthese ................................................. 61
3.5 Transport doorheen mitochondriale membranen ................................................................................... 62
3.5.1 Glycerol-3-phosphate shuttle .............................................................................................................. 62
3.5.2 Malaat-Aspartaat shuttle..................................................................................................................... 62
3
, 3.5.3 ATP-ADP translocase ........................................................................................................................... 63
3.5.4 Phosphate-OH transporter .................................................................................................................. 63
3.5.5 Drijvende kracht achter het membraan transport .............................................................................. 63
3.5.6 Overzicht oxidatieve fosforylatie ......................................................................................................... 64
3.5.7 De volledige oxidatie van 1 glucose levert ongeveer 30 ATP .............................................................. 64
3.6 De regulatie van de cellulaire respiratie .................................................................................................. 64
3.6.1 Respiratoire controle van ATP synthese (oxidatieve fosforylatie) is afhankelijk van ADP concentratie
64
3.6.2 Ontkoppeling van de oxidatieve fosforylatie ....................................................................................... 65
3.6.3 Mitochondriale genen ......................................................................................................................... 65
3.6.4 Mitochondria spelen een sleutelrol in apoptose ................................................................................. 65
4.1 Inleiding.................................................................................................................................................... 66
4.2 De pentosefosfaat weg ............................................................................................................................ 66
4.2.1 De oxidatieve fase (fase1) ................................................................................................................... 67
4.2.2 Conversie van ribulose-5-P naar ribose-5-P en xylulose-5-P ............................................................... 67
4.2.3 Niet-oxidatieve fase (fase2) ................................................................................................................. 67
4.3 Het lot van glucose-6-P ............................................................................................................................ 69
4.3.1 Regulatie pentosefosfaat weg ............................................................................................................. 69
4.3.2 MODUS1: behoefte ribose-5-P >>> behoefte NADPH ......................................................................... 70
4.3.3 MODUS 2: behoefte ribose-5-P = behoefte NADPH ............................................................................ 70
4.3.4 MODUS 3: behoefte ribose-5-P <<< behoefte NADPH ........................................................................ 71
4.3.5 MODUS 4: naast behoefte NADPH ook behoefte ATP ........................................................................ 71
4.4 Rol van pentosefosfaat weg in bescherming tegen oxidatieve stress ..................................................... 72
4.4.1 Detoxificatie van ROS in de erythrocyt ................................................................................................ 72
4.4.2 Gluc-6-P dehydrogenase deficiëntie .................................................................................................... 73
5.1 Inleiding.................................................................................................................................................... 74
5.1.1 Structuur glycogeen ............................................................................................................................. 74
5.1.2 Waarom slaan we glucose op in de vorm van glycogeen? .................................................................. 74
5.2 Glycogeen afbraak ................................................................................................................................... 74
5.2.1 (Glycogeen) Phosphorylase ................................................................................................................. 75
5.2.2 Phosphoglucomutase .......................................................................................................................... 75
5.2.3 Transferase en α-1,6-glucosidase ........................................................................................................ 76
5.2.4 Glucose-6-P phosphatase (lever) ......................................................................................................... 76
5.3 Regulatie glycogeen afbraak .................................................................................................................... 77
5.3.1 Phosphorylase is aanspreekpunt voor de regulatie ............................................................................ 77
5.3.2 Phosphorylase: regulatie door metabolieten ...................................................................................... 77
5.3.3 Phosphorylase: regulatie door covalente modificatie ......................................................................... 78
4
INHOUD
1.1 Inleiding.................................................................................................................................................... 12
1.1.1 Definities .............................................................................................................................................. 12
1.1.2 Korte geschiedenis van de glycolyse ................................................................................................... 12
1.1.3 Glucose als brandstof molecule ........................................................................................................... 12
1.1.4 Fermentatie versus complete oxidatie ................................................................................................ 12
1.1.5 Lot van glucose in verschillende weefsels ........................................................................................... 13
Rode bloedcellen ........................................................................................................................................... 13
Hersenen ....................................................................................................................................................... 13
Hart en spier.................................................................................................................................................. 14
Vetweefsel .................................................................................................................................................... 14
Lever .............................................................................................................................................................. 14
1.1.6 De glycolyse is een katabole pathway ................................................................................................. 15
1.2 Opname van glucose in de cel via transporters ....................................................................................... 15
1.2.1 Gefaciliteerde diffusie: Glucose Transporters (GluT) .......................................................................... 15
1.2.2 Actief transport van glucose: Na+/glucose symporters ....................................................................... 16
1.3 De glycolyse ............................................................................................................................................. 16
1.3.1 Fase 1 glycolyse: Hexokinase ............................................................................................................... 16
Phosphoglucose isomerase ........................................................................................................................... 17
phosphofructokinase (PFK) ........................................................................................................................... 18
1.3.2 Fase 2 glycolyse: Het 6-C suiker wordt gesplitst in 2 3-C fragmenten door aldolase .......................... 18
1.3.3 Fase 3 glycolyse: Energie transformatie .............................................................................................. 18
1.3.4 Het behoud van de redox balans ......................................................................................................... 20
1.3.4.1 het lot van pyruvaat .................................................................................................................... 20
1.3.4.2 ethanol fermentatie .................................................................................................................... 21
1.3.4.3 lactaat fermentatie ..................................................................................................................... 21
1.3.5 Disacchariden als bron van koolhydraten ........................................................................................... 22
1.3.5.1 Galactose metabolisme .............................................................................................................. 22
1.3.5.2 Fructose metabolisme in de lever............................................................................................... 22
1.3.5.3 Overzicht ..................................................................................................................................... 23
1.3.6 Metabole afwijkingen .......................................................................................................................... 23
1.3.4.1 Lactose intolerantie........................................................................................................................... 23
1.3.4.2 Galactosemia’s ............................................................................................................................ 23
1.3.4.3 Fructose intolerantie en aldolase B ............................................................................................ 24
1
,1.4 De glycolytische weg is strikt gecontroleerd ............................................................................................ 24
1.4.1 Duale rol van de glycolyse ................................................................................................................... 24
1.4.2 Enzymen die belangrijk zijn in de regulatie ......................................................................................... 24
1.4.2.1 Situatie in de spier ...................................................................................................................... 25
1.4.2.2 Situatie in de lever ...................................................................................................................... 26
1.4.3 Kanker en glycolyse ............................................................................................................................. 27
1.5 De gluconeogenese .................................................................................................................................. 27
1.5.1 Noodzaak van de gluconeogenese ...................................................................................................... 27
1.5.2 Precursoren voor de gluconeogenese ................................................................................................. 27
1.5.3 Reacties van de gluconeogenese ......................................................................................................... 28
1.5.3.1 Pyruvaat carboxylase .................................................................................................................. 28
1.5.3.2 PEPCK: phosphoenolpyruvaat carboxykinase ............................................................................. 30
1.5.3.3 Omzetting van glycerol naar dihydroxyacetone phosphate ....................................................... 31
1.6 Gluconeogenese en glycolyse worden reciprook geregeld ..................................................................... 32
1.6.1 Glycolyse en gluconeogenese zijn niet tegelijk actief.......................................................................... 32
1.6.2 De balans tussen glycolyse en gluconeogenese in de lever is gevoelig aan de bloed glucose
concentratie ...................................................................................................................................................... 34
1.6.3 Substraat cyclering versterkt de reciproke signalen en produceert warmte door groot ATP verbruik36
1.6.4 Lactaat gevormd door samentrekkende spieren wordt gebruikt door andere organen: Cori cyclus . 36
2.1 Inleiding.................................................................................................................................................... 38
2.2 Omzetting van pyruvaat naar acetyl-CoA ................................................................................................ 39
2.2.1 Het pyruvaat dehydrogenase complex (PDH complex) ....................................................................... 39
2.2.2 Structuur acetyl-CoA............................................................................................................................ 39
2.2.3 Werkingsmechanisme pyruvaat dehydrogenase complex .................................................................. 39
2.2.3.1 E1 : TPP afhankelijke decarboxylatie (rate-limiting step) ........................................................... 40
2.2.3.2 E1: Oxidatie ................................................................................................................................. 40
2.2.3.3 E2: Vorming van acetyl-CoA ........................................................................................................ 41
2.2.3.4 E3: re-oxidatie van dihydrolipoamide ......................................................................................... 41
2.2.3.5 Overzicht ..................................................................................................................................... 41
2.2.4 Structuur van het pyruvaat dehydrogenase complex ......................................................................... 42
2.3 Reacties van de citroenzuurcyclus ........................................................................................................... 42
2.3.1 Citraat synthase vormt citraat uit oxaalacetaat en acetyl-CoA ........................................................... 43
2.3.1.1 Citraat synthase: werkingsmechanisme ..................................................................................... 43
2.3.2 Cis-Aconitase: citraat wordt geïsomeriseerd tot isocitraat ................................................................. 44
2.3.2.1 Cis-Aconitase: werkingsmechanisme .......................................................................................... 44
2.3.3 Isocitraat dehydrogenase : oxidatieve decarboxylatie van isocitraat ................................................. 44
2.3.4 α-ketoglutaraat dehydrogenase complex ........................................................................................... 45
2
, 2.3.5 Succinyl-CoA synthase ......................................................................................................................... 45
2.3.5.1 Succinyl-coA synthase: werkingsmechanisme ............................................................................ 45
2.3.6 Succinaat dehydrogenase .................................................................................................................... 46
2.3.7 Fumarase ............................................................................................................................................. 46
2.3.8 Malaat dehydrogenase ........................................................................................................................ 47
2.4 Regulatie van de citroenzuurcyclus ......................................................................................................... 47
2.4.1 Regulatie van het pyruvaat dehydrogenase complex ......................................................................... 47
2.4.1.1 Allosterisch: productinhibitie ...................................................................................................... 48
2.4.1.2 Covalente modificatie ................................................................................................................. 48
2.4.2 Regulatie van de citroenzuurcyclus ..................................................................................................... 48
2.5 De citroenzuurcyclus als bron van biosynthetische precursor moleculen: amfibool karakter ................ 49
2.6 Metabole aandoeningen .......................................................................................................................... 50
3.1 Inleiding.................................................................................................................................................... 51
3.1.1 Nood aan ATP regeneratie................................................................................................................... 51
3.1.2 Oxidatieve fosforylatie gebeurt in de mitochondriën ......................................................................... 51
3.2 Oxidatieve fosforylatie is afhankelijk van elektronentransport ............................................................... 52
3.2.1 De redox potentiaal en verandering in vrije energie ........................................................................... 52
3.2.2 ∆G voor transport van één H+.............................................................................................................. 53
3.3 De elektronentransportketen .................................................................................................................. 53
3.3.1 De respiratie keten .............................................................................................................................. 53
E’° van de verschillende complexen .............................................................................................................. 53
Ubiquinone (Q) (of CoQ) ............................................................................................................................... 54
Ijzer-zwavelstof centra worden veel gebruikt in de ETK ............................................................................... 54
3.3.2 Complex I: NADH-Q oxidoreductase.................................................................................................... 54
3.3.3 Complex II: Succinaat-Q reductase ...................................................................................................... 55
3.3.4 Complex III: Q-cytochroom c oxidoreductase ..................................................................................... 55
3.3.5 Complex IV: cytochrome c oxidase ...................................................................................................... 57
3.3.6 Toxische derivaten van O2 ................................................................................................................... 58
3.4 De protonengradiënt drijft de synthese van ATP..................................................................................... 58
3.4.1 Verband tussen elektronen transport en ATP synthese ...................................................................... 58
3.4.2 ATP synthase ....................................................................................................................................... 59
3.4.2.1 ATP synthase: Structuur .............................................................................................................. 59
3.4.3 Binding-change mechanisme van Paul Boyer ...................................................................................... 60
3.4.4 De protonen die langs de c-ring stromen zorgen voor ATP synthese ................................................. 61
3.5 Transport doorheen mitochondriale membranen ................................................................................... 62
3.5.1 Glycerol-3-phosphate shuttle .............................................................................................................. 62
3.5.2 Malaat-Aspartaat shuttle..................................................................................................................... 62
3
, 3.5.3 ATP-ADP translocase ........................................................................................................................... 63
3.5.4 Phosphate-OH transporter .................................................................................................................. 63
3.5.5 Drijvende kracht achter het membraan transport .............................................................................. 63
3.5.6 Overzicht oxidatieve fosforylatie ......................................................................................................... 64
3.5.7 De volledige oxidatie van 1 glucose levert ongeveer 30 ATP .............................................................. 64
3.6 De regulatie van de cellulaire respiratie .................................................................................................. 64
3.6.1 Respiratoire controle van ATP synthese (oxidatieve fosforylatie) is afhankelijk van ADP concentratie
64
3.6.2 Ontkoppeling van de oxidatieve fosforylatie ....................................................................................... 65
3.6.3 Mitochondriale genen ......................................................................................................................... 65
3.6.4 Mitochondria spelen een sleutelrol in apoptose ................................................................................. 65
4.1 Inleiding.................................................................................................................................................... 66
4.2 De pentosefosfaat weg ............................................................................................................................ 66
4.2.1 De oxidatieve fase (fase1) ................................................................................................................... 67
4.2.2 Conversie van ribulose-5-P naar ribose-5-P en xylulose-5-P ............................................................... 67
4.2.3 Niet-oxidatieve fase (fase2) ................................................................................................................. 67
4.3 Het lot van glucose-6-P ............................................................................................................................ 69
4.3.1 Regulatie pentosefosfaat weg ............................................................................................................. 69
4.3.2 MODUS1: behoefte ribose-5-P >>> behoefte NADPH ......................................................................... 70
4.3.3 MODUS 2: behoefte ribose-5-P = behoefte NADPH ............................................................................ 70
4.3.4 MODUS 3: behoefte ribose-5-P <<< behoefte NADPH ........................................................................ 71
4.3.5 MODUS 4: naast behoefte NADPH ook behoefte ATP ........................................................................ 71
4.4 Rol van pentosefosfaat weg in bescherming tegen oxidatieve stress ..................................................... 72
4.4.1 Detoxificatie van ROS in de erythrocyt ................................................................................................ 72
4.4.2 Gluc-6-P dehydrogenase deficiëntie .................................................................................................... 73
5.1 Inleiding.................................................................................................................................................... 74
5.1.1 Structuur glycogeen ............................................................................................................................. 74
5.1.2 Waarom slaan we glucose op in de vorm van glycogeen? .................................................................. 74
5.2 Glycogeen afbraak ................................................................................................................................... 74
5.2.1 (Glycogeen) Phosphorylase ................................................................................................................. 75
5.2.2 Phosphoglucomutase .......................................................................................................................... 75
5.2.3 Transferase en α-1,6-glucosidase ........................................................................................................ 76
5.2.4 Glucose-6-P phosphatase (lever) ......................................................................................................... 76
5.3 Regulatie glycogeen afbraak .................................................................................................................... 77
5.3.1 Phosphorylase is aanspreekpunt voor de regulatie ............................................................................ 77
5.3.2 Phosphorylase: regulatie door metabolieten ...................................................................................... 77
5.3.3 Phosphorylase: regulatie door covalente modificatie ......................................................................... 78
4
